The large dielectric breakdown field (>2.2 MV/cm) and the high current density of the two dimensional electron gas (2-DEG) in III-nitride heterojunction semiconductor devices make them attractive for power applications.
One known III-nitride heterojunction power semiconductor device is a high electron mobility transistor (HEMT). A desirable variation of a HEMT is a normally-off HEMT; i.e. a HEMT which does not allow for conduction of current (except for minute leakage current) in the absence of an appropriate voltage to its gate electrode.
FIG. 1 illustrates a normally-off III-nitride power semiconductor device, which includes a III-nitride semiconductor stack 1. Stack 1 includes N-polar GaN layer 2, N-polar AlGaN layer 3, N-polar GaN layer 4, and N-polar AlGaN layer 5, each usually grown using molecular beam epitaxy (MBE).
To fabricate a device according to FIG. 1, stack 1 is grown first, and then AlGaN layer is etched away to define areas for power electrodes (e.g. source and drain electrodes) 6,7. Gate structure 8 which may include a gate insulation and gate electrode stack or a schottky gate electrode is then formed on the remainder of AlGaN layer 5.
AlGaN layer 5 under gate structure 8 pulls the conduction band above the Fermi energy and removes the 2DEG under gate structure 8. As a result the device is rendered normally-off.
While fabricating a device according to FIG. 1, N-polar GaN layer 4 is damaged during the etching of AlGaN layer 5. As a result, the quality of the ohmic contact between power electrodes 6,7 and GaN layer 4 is diminished, resulting in a lesser quality device.